An outboard marine heat exchanger made in accordance with the thus-entitled Fernstrum U.S. Pat. No. 2,382,218 issued Aug. 14, 1945 has for many years successfully solved the cooling problems of marine engines of moderate size and power. More recently, however, such heat exchangers made in accordance with the Fernstrum patent have proved to be inadequate for large engines of high power. Attempts to produce the component thin rectangular hollow tubes have proved unsuccessful because of the inability, under the present state of the extruding art, to extrude tubes of rectangular cross-section from copper or copper alloys wherein the vertical dimension exceeds the horizontal dimension by a ratio of more than three to one, such factors in such tubes of up to five-to-one ratio resulting in a high percentage of defective and therefore wasted tubes. The present invention enables production of composite tubes of rectangular cross-section wherein the ratio of the height to the width can be ten times or more, the welded-together intermediate walls forming a doubly-thick partition or web which serves as a backbone to greatly strengthen the composite tube and to prevent collapse thereof under the pressure of water passing through the tubes.